Soft actuators aim to bridge the gap between rigid machines and soft matter by mimicking the flexibility and compliance of natural muscles and tissues. Stimuli-responsive hydrogels can fit this purpose through their softness and ability for large reversible shape morphing. However, the performance of hydrogel actuators is restricted by their diffusion-limited water transport, which leads to slow responses, and uniform volumetric changes that generate limited actuation forces and strains. Thus, there is a need to develop faster hydrogel actuators that can efficiently convert swelling into actuation force and strain. Anisotropic cellulose nanofibril (CNF) hydrogels offer a route to overcome these limitations by assembling charged fibrils into dense, layered sheets that are reinforced in-plane while remaining compliant in the thickness direction. This architecture redirects water uptake into uniaxial expansion or, under confinement, into high blocking pressure, thereby combining large strain, high force, and rapid response. Here, we establish how the structure and processing of charged CNF networks govern swelling-driven actuation. Specifically, we examine how fibril properties, including aspect ratio and charge density, together with sheet fabrication parameters, including drying conditions and actuator area, control the translation of water uptake into directional strain and force. Optimization of these parameters results in CNF networks that expand uniaxially by 220 times within an hour with initial strain rates of 190–300% s−1, reaching blocking pressures up to 4.9 MPa in less than a minute. These hydrogels are a great step towards hydrogel-based artificial muscles, which have been prevented by the slow response of previously reported stimuli-responsive hydrogels. Further development of fibrillar hydrogel actuators can lead to truly lifelike artificial muscles.

Superabsorbent polymers (SAPs) are essential components in absorption products for food packaging, agriculture, wound dressings, and hygiene. Modern absorption products are designed to rapidly absorb and transport liquids to SAPs, which drains the porous networks and hold liquids under pressure as hydrogels. However, the carbon footprint of these massively used, fossil-based products is high, leading to an urgent need to develop biobased superabsorbents. Although commercial SAPs have an absorption capacity under load that is challenging to surmount, their powder form complicates processing. Thus, biobased alternatives can compete with other advantages, such as intelligently designed self-supporting structures preferably manufactured in sustainable roll-to-roll processes. As a pioneering step, this study presents all-cellulose superabsorbent heterostructures prepared by combining macroporous fiber-based aerogels with highly swelling cellulose nanofibril (CNF) sheets. The aerogel rapidly absorbs 30 g g-1 of liquid, which is rapidly transferred to the CNF sheets with a maximum capacity of 246 g g-1, holding liquids at pressures of up to 0.9 MPa. The heterostructure is also equipped with a simple, sustainable conductometric water-uptake sensor to follow the liquid uptake and saturation level. Using unmodified raw materials from the forest industry in a scalable process with the potential for roll-to-roll manufacturing makes these all-cellulose heterostructures a competitive alternative to commercial SAPs in a carbon-neutral society.

Expanding our understanding of how cellulose fibers and fibrils interact with water and its effect on their inherent properties is needed to optimize their utilization in the making of novel bio-based materials, but also useful in more traditional products (pulp, paper, and packaging).

The overall objective of the work in this thesis was to deepen the understanding of drying-induced structural changes and cellulose-water interactions using cellulosic model materials. Cellulose nanofibrils (CNFs) were employed as they present a distinct advantage with their defined geometry and controlled surface chemistry compared to macroscopic cellulose fibers.

The first part considers the fundamental interactions of CNFs in contact with water and by water removal, and is devoted to identifying the molecular mechanisms behind the process known as hornification. This was done by studying the exposure of CNF sheets to different heat treatments to establish a connection between their reswelling properties, chemical and structural characteristics, and mechanical behavior. The findings indicate that hornification is governed by non-covalent interactions and that the diffusion of water back into a hornified CNF network is kinetically limited. Furthermore, the influence of fibril aspect ratio and chemical functionality on the mechanical properties of wet fibrillar networks was studied. Fibrils were prepared from fibers with different hemicellulose content. It was found that longer fibrils formed stiffer and more ductile materials, owing to a longer-range and more uniform distribution of stress transfer. Additionally, high aspect ratio fibrils form networks capable of holding larger amounts of water. It was also possible to elucidate the influence of aspect ratio on the network formation, where long and short fibrils form networks with different topologies. These results were integrated into a mechanical network model to present an improved elastoplastic description of the network properties.

The second part of the thesis presents potential applications where control of the water uptake in the fibrillar networks is required. Anisotropic fibrillar hydrogels were prepared to function as actuators and superabsorbents. With the help of the knowledge built in the first part of the work, the water uptake in the fibrillar networks could be maximized. This resulted in CNF hydrogel actuators far surpassing conventional hydrogel used in actuation performance and integration of CNF sheets in a superabsorbent heterostructure, where the most strongly immobilized water can be retained at high pressures. 

Att utvidga vår förståelse för hur cellulosafibrer och fibriller interagerar med vatten och dess effekt på deras inneboende egenskaper är nödvändigt för att optimera deras användning vid tillverkning av nya biobaserade material, men även i traditionella produkter (massa, papper och förpackningar).

Det övergripande målet med arbetet i denna avhandling var att fördjupa förståelsen för strukturella förändringar orsakade av torkning och cellulosa-vatten interaktioner med hjälp av cellulosabaserade modellmaterial. Cellulosa nanofibriller (CNFs) användes då de har en fördel med sin definierade geometri och kontrollerade ytkemi i jämförelse med makroskopiska cellulosafibrer. 

Den första delen avhandlar de grundläggande interaktionerna mellan CNFs i kontakt med vatten och efter vattenborttagning, och ägnas åt att identifiera de molekylära mekanismerna bakom processen som kallas hornifiering. Detta gjordes genom att studera CNF-ark som utsatts för olika värmebehandlingar för att fastställa en koppling mellan deras svällningsegenskaper, kemiska och strukturella egenskaper samt mekaniska beteende. Resultaten indikerar att förhorning styrs av icke-kovalenta interaktioner och att diffusionen av vatten tillbaka in i ett förhornat CNF-nätverk är kinetiskt begränsad. Sedan studerades inflytandet av fibrillernas längd-bredd förhållande och kemisk funktionalitet på de mekaniska egenskaperna hos våta fibrillnätverk. Fibriller framställdes från fibrer med olika hemicellulosahalt. Resultaten visade att längre fibriller bildade styvare och mer töjbara material, tack vare en längre och mer jämn fördelning av belastningsöverföring. Dessutom bildar fibriller med högt längd-bredd förhållande nätverk som kan hålla större mängder vatten. Det var också möjligt att förklara inflytandet av längd-bredd förhållande på nätverksbildningen, där långa och korta fibriller bildar nätverk med olika topologier. Dessa resultat integrerades i en mekanisk nätverksmodell för att presentera en förbättrad elastoplastisk beskrivning av nätverksegenskaperna.

Den andra delen av avhandlingen presenterar potentiella tillämpningar där kontroll av vattenupptaget i fibrillnätverken krävs. Anisotropa fibrillära hydrogeler framställdes för att fungera som aktuatorer och superabsorbenter. Med hjälp av kunskapen som byggts upp i första delen av avhandlingen kunde vattenupptaget i fibrillnätverken maximeras. Detta resulterade i CNF hydrogelaktuatorer som långt överträffade konventionella hydrogeler i aktuatorprestanda, och integration av CNF-ark i en superabsorbent heterostruktur, där det starkast bundna vattnet kan behållas vid höga tryck.

Nanocellulose has emerged as a widely utilized building block in nanostructured materials due to its availability, sustainability, large surface area, and high stiffness and aspect ratio. The wet or dry elastoplastic properties of these materials are determined by the fibrils' stiffness, chemical properties, hemicellulose content, and the number of fibril contacts. However, the specific contributions and relative importance of each factor remain unclear. Therefore, this work was devoted to systematically comparing the material properties of gels, aerogels, and wet and dry sheets prepared from CNFs with different aspect ratios, chemical functionality, and hemicellulose content. The fibrils were prepared by chemical and mechanical processing of different pulps. By preserving the native structure as much as possible, higher aspect ratio fibrils can be obtained, which allows for the development of more mechanically robust materials. The results demonstrate that higher aspect ratios lead to more interconnected networks at a lower solids concentration, resulting in a more evenly distributed stress and longer-range stress transfer, yielding stiffer and more ductile materials. The most important finding was that the aspect ratio influences the network formation, resulting in different network topologies. The results were also compared to earlier published data and integrated into a theoretical beam-bending model for a complete elastoplastic description of the network properties, including the influence of fibril aspect ratio and chemical functionality. This information improves our understanding and description of nanofibril networks for which general models have been missing. It can be used to optimize nanofibril preparation and, hence, the resulting eco-friendly materials.

Tunable dynamic networks of cellulose nanofibrils (CNFs) are utilized to prepare high-performance polymer gel electrolytes. By swelling an anisotropically dewatered, but never dried, CNF gel in acidic salt solutions, a highly sparse network is constructed with a fraction of CNFs as low as 0.9%, taking advantage of the very high aspect ratio and the ultra-thin thickness of the CNFs (micrometers long and 2–4 nm thick). These CNF networks expose high interfacial areas and can accommodate massive amounts of the ionic conductive liquid polyethylene glycol-based electrolyte into strong homogeneous gel electrolytes. In addition to the reinforced mechanical properties, the presence of the CNFs simultaneously enhances the ionic conductivity due to their excellent strong water-binding capacity according to computational simulations. This strategy renders the electrolyte a room-temperature ionic conductivity of 0.61 ± 0.12 mS cm−1 which is one of the highest among polymer gel electrolytes. The electrolyte shows superior performances as a separator for lithium iron phosphate half-cells in high specific capacity (161 mAh g−1 at 0.1C), excellent rate capability (5C), and cycling stability (94% capacity retention after 300 cycles at 1C) at 60 °C, as well as stable room temperature cycling performance and considerably improved safety compared with commercial liquid electrolyte systems.

Fibrillar hydrogels are remarkably stiff, low-density networks that can hold vast amounts of water. These hydrogels can easily be made anisotropic by orienting the fibrils using different methods. Unlike the detailed and established descriptions of polymer gels, there is no coherent theoretical framework describing the elastoplastic behavior of fibrillar gels, especially concerning anisotropy. In this work, the swelling pressures of anisotropic fibrillar hydrogels made from cellulose nanofibrils were measured in the direction perpendicular to the fibril alignment. This experimental data was used to develop a model comprising three mechanical elements representing the network and the osmotic pressure due to non-ionic and ionic surface groups on the fibrils. At low solidity, the stiffness of the hydrogels was dominated by the ionic swelling pressure governed by the osmotic ingress of water. Fibrils with different functionality show the influence of aspect ratio, chemical functionality, and the remaining amount of hemicelluloses. This general model describes physically crosslinked hydrogels comprising fibrils with high flexural rigidity - that is, with a persistence length larger than the mesh size. The experimental technique is a framework to study and understand the importance of fibrillar networks for the evolution of multicellular organisms, like plants, and the influence of different components in plant cell walls.

The fundamental understanding concerning cellulose-cellulose interactions under wet and dry conditions remains unclear. This is especially true regarding the drying-induced association of cellulose, commonly described as an irreversible phenomenon called hornification. A fundamental understanding of the mechanisms behind hornification would contribute to new drying techniques for cellulose-based materials in the pulp and paper industry while at the same time enhancing material properties and facilitating the recyclability of cellulose-rich materials. In the present work, the irreversible joining of cellulose-rich surfaces has been studied by subjecting cellulose nanofibril (CNF) films to different heat treatments to establish a link between reswelling properties, structural characteristics as well as chemical and mechanical analyses. A heating time/temperature dependence was observed for the reswelling of the CNF films, which is related to the extent of hornification and is different for different chemical compositions of the fibrils. Further, the results indicate that hornification is related to a diffusion process and that the reswellability increases very slowly over long time, indicating that equilibrium is not reached. Hence, hornification is suggested to be a kinetically limited phenomenon governed by non-covalent reversible interactions and a time/temperature dependence on their forming and breaking.

Metal–organic frameworks (MOFs) are hybrid porous crystalline networks with tunable chemical and structural properties. However, their excellent potential is limited in practical applications by their hard-to-shape powder form, making it challenging to assemble MOFs into macroscopic composites with mechanical integrity. While a binder matrix enables hybrid materials, such materials have a limited MOF content and thus limited functionality. To overcome this challenge, nanoMOFs are combined with tailored same-charge high-aspect-ratio cellulose nanofibrils (CNFs) to manufacture robust, wet-stable, and multifunctional MOF-based aerogels with 90 wt% nanoMOF loading. The porous aerogel architectures show excellent potential for practical applications such as efficient water purification, CO2 and CH4 gas adsorption and separation, and fire-safe insulation. Moreover, a one-step carbonization process enables these aerogels as effective structural energy-storage electrodes. This work exhibits the unique ability of high-aspect-ratio CNFs to bind large amounts of nanoMOFs in structured materials with outstanding mechanical integrity—a quality that is preserved even after carbonization. The demonstrated process is simple and fully discloses the intrinsic potential of the nanoMOFs, resulting in synergetic properties not found in the components alone, thus paving the way for MOFs in macroscopic multifunctional composites.